Yes, I carry the XR VCO board (we call it the TH-102 "El Cerrito"), and the VCLFO (the TH-301 "Cucamonga"). I also have a few 3080 and 8038 chips in stock. I am working on getting some reliable XR-2206 chips, but that seems to be a bit tougher to nail down. I bought a bunch that I thought were good NOS, but they only seem to work at lower supply voltages (10 or 11 volts instead of 15). Still experimenting with those, but in the meantime, Jameco carries the 2206 for about $6. Anyway, give us a shout at magsmoke@gmail.com if you want more info.

Concerning the power supply, would I be able to just use a standard AC to DC power supply instead of the power supply design included in the booklet that supplies the +/-15V, +5V, and ground or do the wave rectifier circuits included make it imperative to use the included design?

Also, I'm thinking about using a microcontroller to simplify the modules but I'm not sure if this would interrupt the circuits, especially where the pots are concerned..

Any suggestions?

I kind of want to use the microcontroller to also change up the path sequence of the audio signal's flow but, that'll be later in the design process..haha

Another thing about the bypass caps, these caps aren't necessarily connected to anything besides each other and the voltage sources/grounds right? Just placed in close proximity to prevent field noise?

A normally closed jack, has a third connector, and if there is no jack inserted, whatever goes to that connector is then routed to the tip input. In this doc, that's indicated with the arrow pointing at the jack's tip. So on the ADSR for example, you have 15V to the gate unless there's a plug inserted in which case it takes the gate signal from the plug. For the ADSR this lets it operate in a "just triggered" mode. Similarly, the normalling of the trigger input lets it operate in "just gated" mode.

You can use a normally closed switch in place of a normally open one, you just don't use that third connector.

There are lots of sources for these, and they vary widely in expense, so I don't know that I can make any good recommendation there. At this point I usually buy a set of normally closed Switchcraft jacks whenever there's a Group Buy over on muffwiggler.com. I have no idea who they come from originally, I would guess Mouser. The Switchcrafts are somewhat standard in the DIY space and are used by some (most?) of the major module vendors.

A normally closed jack, has a third connector, and if there is no jack inserted, whatever goes to that connector is then routed to the tip input. In this doc, that's indicated with the arrow pointing at the jack's tip. So on the ADSR for example, you have 15V to the gate unless there's a plug inserted in which case it takes the gate signal from the plug. For the ADSR this lets it operate in a "just triggered" mode. Similarly, the normalling of the trigger input lets it operate in "just gated" mode.

You can use a normally closed switch in place of a normally open one, you just don't use that third connector.

There are lots of sources for these, and they vary widely in expense, so I don't know that I can make any good recommendation there. At this point I usually buy a set of normally closed Switchcraft jacks whenever there's a Group Buy over on muffwiggler.com. I have no idea who they come from originally, I would guess Mouser. The Switchcrafts are somewhat standard in the DIY space and are used by some (most?) of the major module vendors.

Thanks man.

Do analog synths normally output a mono audio signal? I know they are typically monophonic but aren't the outputs stereo?

I just finished a compact stripboard layout of the VC Phase Shifter... let me know if you are interested...

Cool. I haven't tried the phase shifter yet, but it looks interesting. Let us know what you think of it. Share a sound sample if you can.

Hey Tim,

I got around to recording some samples of the phase shifter today. I felt a little obliged to record these, because I added the triangle portion of your 8K LFO as a piggyback PCB The only other deviation from the original are 560pF caps for the filters (instead of 470pF). Not sure if/how that affects the sound...

This is my first phaser, so I can't really compare it, but I like it. I think the resonance/feedback could be a bit stronger - maybe this can be modified... Apart from classic phasing sounds, you can get some interesting VCA-like phase cancellation out of it. Overall it is a more complex module than I expected. I guess I need some more time to play with it.

A sophisticated MIDI->CV will output in addition to trigger and gate, not only the base CV, but also Velocity, possibly Modulation, and either incorporate pitch bend into the CV value or output pitch bend separately as well.

Portamento is more commonly known as "glide" or "lag" in most of the stuff I've read. It's essentially done by a separate processor, like the one included on this utility board (lower right corner, in this case marked as portamento ):

Do analog synths typically only have one vco? How does the circuit change when incorporating multiple vcos and how do they interact with the CV keyboard?

How does an envelope generator that is routed to multiple modules (vcf, vca, vco), communicate? Won't the envelope generator cause the particular setting of ADSR to be sent to all those modules at the same time or are the ADSR characteristics of each module controlled separately?

Of course, I'm referring to the Thomas Henry designs but a general answer will suffice.

Don't mistake my willingness to answer with what I know for lots of experience or expertise though . I've only been doing this stuff for just about a year. My only advantage is a degree in electronics that sat fallow for 24 years before I started doing this.

chieffrancis wrote:

More questions..

Do analog synths typically only have one vco? How does the circuit change when incorporating multiple vcos and how do they interact with the CV keyboard?

How does an envelope generator that is routed to multiple modules (vcf, vca, vco), communicate? Won't the envelope generator cause the particular setting of ADSR to be sent to all those modules at the same time or are the ADSR characteristics of each module controlled separately?

I think perhaps one question that's lurking behind all this is what do you mean by "analog synths" . There's a pretty wide range out there, from purpose built keyboards to modulars. Thomas' book of course is referring to modulars, so I have been answering mostly from that perspective.

It's a rare modular that has only one VCO. You'll note there are two in Thomas' book, and an LFO as well (not voltage controlled, but still an oscillator). Referring to the patchover scheme at the end will possibly enlighten some on signal routing, although I would say that was one of the more confusing things to me until I'd read it several times and understood the whole arrow == normalling convention and what normalling actually meant.

One thing that is omitted or less clear in Thomas' book is signal routing. There are a lot of ways that it could be accomplished here. I'm not sure what his assumptions were, but if I had to guess I'd say it looks like he expected someone building this circuits to make a somewhat monolithic semi-patchable synth with them rather than individual modules, but that is MY INTERPRETATION, so I make no claim as to knowing his actual thoughts on the matter. By semi-patchable I mean that there would be a default routing that matches the patchover scheme, using normalling jacks (normally closed), which could then be patched differently on the front panel. This is how the old ARP 2600 worked, among others, and there are some realllly good youtube videos demonstrating the ARP that I would recommend to help get a good understanding of how normalling can work, among other things.

I've built several of the AS4t21C circuits (or variations) as individual modules, and I needed multiples (aka mults, aka buffers) to accomplish the patchover scheme (and I still need to go back to it and figure out what I did wrong, because just plugging it up didn't work as I expected it to ). For my multiples I used a variation on the Music From Outer Space distribution circuits that I linked to above. A mult can either be passive, in which case it's 4 jacks with their tips connected and their grounds connected, or it can be buffered like the MFOS circuits where there are op amp drivers between the input and output--which gives other options as well, like adding in offset voltages or adding glide, as Ray's circuits do.

What I actually built used the trigger mults unchanged, only added offset voltage to one set of CV mults, moved the glide circuit off by itself onto a different module, and added in a sum/difference circuit I got from elsewhere on the net. But I'm off in the weeds now, aren't I?

So, how you use the keyboard CV/gate/trigger depends on what exactly you want to do. It would be common to route the keyboard CV to both VCOs (as in the patchover scheme). In Thomas' suggested setup, the CV goes to each VCO, which would let you tune them to an interval or have them interact in other ways. You'll note that the ADSR goes to 5 (!) different points, controlling not only characteristics of each VCO, but filter cutoff, VCA amplitude, and tuned noise frequency.

In the base design, yes, one envelope is sent to all the other modules, there are not separate envelopes for each module.

As a side note, I built the ADSR as a dual unit, using a 556 dual timer chip, and with a switch to allow the two EGs to be ganged from a single set of gate/trigger inputs. I used someone else's project as inspiration and crib notes, but I cannot for the life of me find it at this moment. But that has been very helpful so I can generate different envelopes for different modules.

Another option would be to use a more simple design; hexinverter.net has a very nice PCB for a quad A/R (attack release, really an ASR, attack sustain release) envelope generator, he calls it the postman.

yes, that is the standard for 5U modulars, 1/4" mono jacks and what would be instrument cables if they were much longer. Typical lengths range from 1 to 3 feet, depending, which wouldn't give you much room for an instrument....

There are other standards of course. Buchla and Serge use banana jacks and banana cables, Eurorack systems use 1/8" jacks (like your standard headphone jack, but mono).

yes, that is the standard for 5U modulars, 1/4" mono jacks and what would be instrument cables if they were much longer. Typical lengths range from 1 to 3 feet, depending, which wouldn't give you much room for an instrument....

There are other standards of course. Buchla and Serge use banana jacks and banana cables, Eurorack systems use 1/8" jacks (like your standard headphone jack, but mono).

I suppose a 1/4" jack would be sufficient for all the inputs/outputs for the modules, for the sake of consistency and familiarity. Would I also be able to incorporate multiple outputs, say, a speaker out as well as an 1/8" AUX output or 1/4" output similar to that of a guitar?

If I used 1/4" jacks for my modules, would that make the signal routing between the modules easier to mix in sequence as well as easily attach modules for which specific output parameters are marked specific to inputs on other modules i.e. linear FM, PWM, and ADSR connections to VCO, VCA, and VCF?

I would want an amplifier to drive speakers, but you could make an amp module easily. Anything with inputs, outputs and controls can be put behind an appropriately sized panel and made into a module. There are folks who do this with guitar pedals, for example.

It is possible to have mults that cross the formats, for interconnecting them. I could have a 1/4" in and bananas and 1/8" jacks out. And even more 1/4"s as well.

For example, if I had 1/4" inputs and outputs on every module, (VCO, VCA, VCF, ADSR, Noise, etc.), I would be able to create a signal path using instrument cables in whatever order I please right? As long as the voltage controlled oscillators, voltage controlled amplifier, and some arbitrary speaker or output, I should be able to get sound coming out right?

Also, concerning the keyboard, I am trying not to spend over $500 total for this project (hardware), so getting a MIDI to CV converter is probably out of the question since I've only seen those that are above $100. I know control voltage is decided by which keys are pressed on a CV keyboard but how are the gates and triggers done? Are these signals generated by the keyboard and sent to the VCO, as well as other modules through the same key press that sends the CV signal?

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